DGSI. Durham Geo Slope Indicator

Grout Mixes for Piezometers

Grout Mix for Hard and Medium Soils
Materials Weight Ratio by Weight
Portland cement 94 lb (1 bag) 1
Water 30 gallons 2.5
Bentonite 25 lb (as required) 0.3

Mix cement with water first. Then mix in the bentonite. Adjust the amount of bentonite to produce a grout with the consistency of heavy cream. If the grout is too thin, the solids and the water will separate. If the grout is too thick, it will be difficult to pump. Read notes below.

Grout Mix for Soft Soils
Materials Weight Ratio by Weight
Portland cement 94 lb (1 bag) 1
Water 75 gallons 6.6
Bentonite 39 lb (as required) 0.4

Mix cement with water first. Then mix in the bentonite. Adjust the amount of bentonite to produce a grout with the consistency of heavy cream. If the grout is too thin, the solids and the water will separate. If the grout is too thick, it will be difficult to pump. Read notes below.

Notes

  • Drillers are accustomed to mixing water and bentonite first, but this will not allow you to control the water-cement ratio. Mix water and cement first. Then add bentonite. There is no particular amount of bentonite that you must add. The thickness of the grout varies with water, temperature, and agitation, so the amount of bentonite required will vary.
  • The quantity of the third ingredient to be added must be adjusted to obtain a suitable consistency. A watery mix is NOT acceptable. A mix that is too thick cannot be pumped. The Marsh funnel number of the liquid grout should be about 55 seconds +/- a few seconds.
  • If you mix cement and water first, the amount of bentonite has to be adjusted. The amount is rarely ever equal to exactly 25 lbs. The final quantity also varies with the type of bentonite used, the method of mixing and the pH of the water.
  • If you mix bentonite and water first, the mix usually gets so thick with one bag of cement that it cannot be pumped. There is also a high risk of a flash-set. The cement content must usually be lowered. That results in a higher w/c ratio, lower strength and probably a higher permeability.
  • If the mix is left too watery, not only do you get shrinkage, but segregation occurs: cement on the bottom, then bentonite and water on top (bleed). This is not acceptable.
  • Need more information? Visit our References and Papers page.